1. Field of the Invention
The present invention relates to a SiC (silicon carbide) substrate and a method of manufacturing the SiC substrate and, more particularly, to a method of manufacturing a SiC substrate in which at least one surface is polished.
2. Description of the Related Art
Recently, there has been a growing demand for lasers and light emitting diodes which use GaN-base semiconductors as light emitting layers and can emit light having a short wavelength, such as the ultraviolet region and blue color. These types of lasers and light emitting diodes are expected to be used as light sources for recording information at high recording densities on optical disks and reproducing information therefrom or light sources for displaying images in full color or for use in illumination. In general, it is difficult to cause a GaN-base semiconductor to grow into the shape of a large single crystal ingot having few crystal defects. For this reason, techniques for epitaxially growing a GaN-base semiconductor layer on a sapphire single crystal substrate or a SiC single crystal substrate are receiving attention and a sapphire single crystal substrate or a SiC single crystal substrate on which a GaN-base semiconductor layer is to be formed is sought after.
A SiC single crystal substrate is demanded also as a substrate for forming a high-quality SiC semiconductor layer. Because a SiC semiconductor has a wide band gap, a large dielectric breakdown electric field and a large thermal conductivity in comparison with a GaAs semiconductor, research and development have been carried out to form high-quality SiC semiconductor layers on a SiC single crystal substrate and to realize semiconductor elements operating at high temperatures and power semiconductor elements having a high breakdown voltage. In addition, in the semiconductor process, dummy wafers made of SiC are demanded because these wafers have excellent heat resistance, high thermal conductivity, high-temperature strength, low thermal expansion, wear resistance, etc.
A sapphire single crystal substrate or a SiC single crystal substrate for such applications is required to provide high working accuracy in the flatness of the substrate, the smoothness of the substrate surface, etc. However, generally a sapphire single crystal or SiC has high hardness and excellent corrosion resistance, and hence the workability of manufacturing such a substrate is bad and it is difficult to obtain a sapphire single crystal substrate and a SiC substrate having high working accuracy.
In particular, as described in Japanese Laid-Open Patent Publication No. 55-20262, when an ingot of sapphire single crystal is cut and lapped and its surface is then mirror finished, a work-affected layer in which work strains have been generated remains on the back surface, posing the problem that the substrate warps.
For this reason, when photolithography is performed on such a substrate, there arises some problems in that it becomes impossible to perform vacuum chucking of the substrate by an exposure device, etc., and that the accuracy of exposure worsens due to a poor flatness of the substrate, and so on. Furthermore, when a thin layer of metal, ceramics, etc. is formed on such a substrate in which a work-affected layer remains, the problem that the substrate breaks because of the addition of the stresses of the thin film to the residual stresses of the substrate arises.
For this reason, the Japanese Laid-Open Patent Publication No. 55-20262 discloses a technique which involves immersing a sapphire single crystal substrate in heated phosphoric acid or potassium hydroxide solution and removing a work-affected layer remaining in the substrate by dissolving the work-affected layer thereby to eliminate a warp of the substrate.
However, in the case of a SiC substrate, it is impossible to dissolve SiC with heated phosphoric acid or potassium hydroxide solution. Although fused alkalis which are heated to not less than 300° C. are known as solutions which dissolve SiC, large-scale equipment is necessary for safely handling high-temperature fused alkalis.
The Japanese Laid-Open Patent Publication No. 55-20262 discloses that ion sputtering and ion etching may also be adopted as other processes for removing the work-affected layer of a sapphire single crystal substrate. However, these processes involve performing the etching of a substrate surface by utilizing the physical energy of ions of argon, etc., which are accelerated by causing these ions to collide against the substrate surface. Thus, these processes have the problem that the etching rate is low.
Furthermore, because the melting point of SiC is not less than 2000° C., it is necessary to heat a SiC substrate to not less than 1600° C. in order to remove work strains by annealing. Large-scale equipment is necessary for subjecting the SiC substrate to heat treatment at such a high temperature.